That is the only way it is legal. This was an export directive from the Commerce department, using authority granted by Congress to control exports of tech. If they just told Anthropic to shut down a model without invoking an emergency order of export, there would be no legal authority for such an order.
Keep in mind that for all the troubles and trauma caused by the current USA government, they are really good at manipulating the legal system to get their way. This is just another example of it.
Ok, but then the government's argumentation doesn't make sense? If the reason for shutting it down is that it's dangerous, only shutting it down for non US citizens doesn't solve the problem.
This sort of thing has been done before - for encryption - and many physical products like missiles. If you sell a missile to someone in the US they're accountable to US law but if you export one then US law stops at the border and nothing stops them firing it at Washington with impunity.
But foreigners in the country are effected as well.